In radiography which acquires an image by applying radiation to an object and detecting radiation having passed through the object, digital radiography (DR) which acquires an image by converting the detected radiation into an electrical signal is popular. In general, in DR, a flat panel detector (FPD) is used. The FPD includes a light receiving element having two-dimensionally arranged pixels and a scintillator layer formed on a surface of the light receiving element.
Depending on the application, in most cases, a wide image pickup area of several tens of centimeters or more per side is required for the FPD, and thus, the scintillator layer to be formed is required to have a large area. Therefore, the scintillator layer is formed by using vacuum deposition which enables formation of a large-area layer or an application method of applying a binding agent having scintillator particles dispersed therein.
In particular, a scintillator layer formed by depositing cesium iodide (CsI) has an advantage that a high positional resolution may be obtained because when cesium iodide is grown as needle crystals, crosstalk is suppressed by light guiding in the needle crystals. However, adjacent CsI needle crystals are liable to adhere to each other, and this adhesion degrades the waveguiding property of scintillation light, to thereby decrease the resolution of a radiation detector.
In PTL 1, there is proposed that a structure including two crystal phases having different refractive indices be used as a scintillator layer. This structure is a phase separation crystal including a plurality of first phases (cylinder phases) having unidirectionality, and a second phase (matrix phase) present on the periphery of each of the first phases, and scintillation light emitted by the first phases or the second phase is confined in the phase having a higher refractive index. With this, the scintillation light is guided in an extending direction of the first phases. Therefore, when this structure is used as a scintillator layer, a high resolution can be obtained.
In the above-mentioned structure, the second phase is present between the first phases, and hence the adhesion of the first phases is less liable to occur as compared to the adhesion between the CsI needle crystals. Thus, it is conceivable that a higher resolution is obtained through use of the phase separation crystals as the scintillator layer instead of the CsI needle crystals.
In order to produce such a scintillator layer including a phase separation crystal in which two crystal phases having different refractive indices are completely separate from each other, it is conceivable to employ a technique of micromachining a scintillator crystal, a technique of separating two phases of eutectic composition in one axial direction and growing the two phases, or the like.
However, it is technically difficult to obtain by those techniques a phase separation crystal having a large area of several tens of centimeters per side. In order to use a phase separation crystal as a scintillator layer of an FPD, it is necessary to spread (tile) a plurality of phase separation crystals, each of which is processed to have a predetermined shape, over a surface of a light receiving element in order to secure a large image pickup area.
In tiling, slight clearance is formed between adjacent phase separation crystals due to limitations on the processing accuracy. This clearance reaches about several μm to about several tens of μm depending on the processing accuracy. When the clearance is filled with a medium having an appropriate refractive index of reducing reflection and scattering of light as in PTL 1, an amount of an X-ray entering pixels arranged in the clearance can be increased to reduce the influence on an X-ray image.